DVM circuits

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  • barry wilkins
    Senior Member
    • Aug 2009
    • 172

    #1

    DVM circuits

    Unforunately I damaged one of my DVM,s when working on some power supplies
    measuring some high voltages ,because there was some rf on the dc I was measuring.
    It still works but is noisey and it will not measure diodes, I would like to fix it if possible.
    Does any one have any circuits for DVM,s

    thanks

    Barry Wilkins
  • redwire
    Badcaps Legend
    • Dec 2010
    • 3902
    • Canada

    #2
    Re: DVM circuits

    There's lots of DVM schematics out there, if you have a make/model or pictures of the board it can help figure out the DVM chip used and what parts might be damaged.

    Usually a low cost DVM overloaded (by voltage) will damage its input protection clamp diodes, so you always get a low reading on ohms or diode test, when no leads are connected. Just speaking generally.

    Comment

    • barry wilkins
      Senior Member
      • Aug 2009
      • 172

      #3
      Re: DVM circuits

      Hi attached some pictures cant see any makers name on it .Fixed the noise problem by cleaning the switch ,just seems a problem trying to measures diodes ,very low voltage applied to the device .
      There are two op amps ,dual and a quad .
      Could it be a problem with the LM324 ?
      Bought a new one from Amazon that had 1000 V on it ,was rubbish didnt have any sockets fitted for the transistors, so it returned it.
      Got a different one which is better.

      Barry
      Attached Files

      Comment

      • stj
        Great Sage 齊天大聖
        • Dec 2009
        • 30965
        • Albion

        #4
        Re: DVM circuits

        Originally posted by barry wilkins
        Bought a new one from Amazon that had 1000 V on it ,was rubbish didnt have any sockets fitted for the transistors, so it returned it.

        Barry
        lol
        there is a running joke that any meter with a transistor (HFE) socket on the front is junk aimed at hobbyists.

        Comment

        • redwire
          Badcaps Legend
          • Dec 2010
          • 3902
          • Canada

          #5
          Re: DVM circuits

          The PC board is "MX-480D-B" but no hits.
          I don't see the usual two-transistor clamp or a PTC thermistor. Low cost DMM's sometimes have missing protection circuits and they easily die.
          I would (using another multimeter) test the six black diodes. Otherwise, all that remain are the LM324 and the big IC.

          hFE socket is also supported by the ANENG/Dreamtech chips, is in DT830 and even Harbor Freight freebies. I don't mind it.

          Comment

          • R_J
            Badcaps Legend
            • Jun 2012
            • 9535
            • Canada

            #6
            Re: DVM circuits

            It is likely a Maxcom MX480 multimeter, or copy. There is a blue PTC1 located close to the board number
            Last edited by R_J; 01-22-2021, 03:25 PM.

            Comment

            • zx8401
              Phantom Fiddler
              • Aug 2013
              • 49
              • uk

              #7
              Re: DVM circuits

              Looks like a dt890 to me, there are a few versions A B C D E F.

              Comment

              • barry wilkins
                Senior Member
                • Aug 2009
                • 172

                #8
                Re: DVM circuits

                Hi thanks to all of you for the info ,I will look at it in a few days .
                Not too bothered about fixing it as I have quite a few other meters
                1 I keep my car 1 in my tool box 1 in my caravan, 3 in my little workshop , a TEK scope and a Hantek scopemeter.

                regards

                Barry

                Comment

                • barry wilkins
                  Senior Member
                  • Aug 2009
                  • 172

                  #9
                  Re: DVM circuits

                  I could see there was something wrong around the TL 062 opamp ,so I removed it and replaced it with an 072 ,replaced the diodes and capcitors at the same time. A bit more investigation found R 36 was O/C.

                  Barry
                  Attached Files

                  Comment

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